Support %define "KEY" {VALUE}.

* src/scan-code.h, src/scan-code.l (translate_action)
(translate_rule_action, translate_symbol_action, translate_code):
Return char *, not const char *.
* src/parse-gram.y (declaration): Rename as...
(prologue_declaration): this.
(string_content): Remove this nonterminal, use STRING.
(braceless, content, content.opt): New nonterminal.
Use them.
(%define): Now accept content.opt, i.e., accept also BRACED_CODE
as value.
* src/scan-gram.l (getargs.h): Don't include it.

src/scan-gram.l

@@ -37,7 +37,6 @@
 
 #include "complain.h"
 #include "files.h"
-#include "getargs.h"    /* yacc_flag */
 #include "gram.h"
 #include "quotearg.h"
 #include "reader.h"